For the crust: Pulse the flour, sugar, salt and baking powder in the bowl of a food processor until combined. Add half of the butter and pulse until the mixture resembles fine cornmeal. Add the remaining butter and continue pulsing until no pieces are larger than peas. Add 2 tablespoons of the ice water and pulse until combined. Add just enough of the remaining 1 to 2 tablespoons ice water, a little at a time, and pulse until the dough just comes together and holds its shape when squeezed. Turn the dough out onto a large sheet of plastic wrap, gather into a disc and wrap well. Refrigerate until firm, at least 1 hour or up to overnight.

Roll out the dough on a lightly floured surface to an 11-inch circle and fit it into a 9-inch pie plate. Crimp the edges and refrigerate the dough until chilled, about 20 minutes.

Preheat the oven to 350 degrees F. Prick the bottom and sides of the crust several times with the tines of a fork. Fit a piece of aluminum foil over the crust and fill the foil with pie weights or dry beans. Bake until the edges of the crust are golden, 20 to 25 minutes. Remove the foil and weights, return the crust to the oven and continue to bake until the bottom of the crust is lightly golden and no longer shiny, 10 to 15 minutes longer. Cool completely.

For the filling: While the crust cools, use a fork to prick the yams all over. Place them on a rimmed baking sheet and roast with the oven still at 350 degrees F until very soft, 45 minutes to 1 hour. Set aside until cool enough to handle. Peel and mash until very smooth; you should have about 2 cups. Transfer the mash to a medium bowl and add the cream, brown sugar, vanilla, allspice, ginger, salt and eggs. Whisk just until smooth. Gently tap the bottom of the bowl a few times to release any air bubbles.

Pour the filling into the cooled crust and bake until the center of the pie is set and no longer shiny, 1 hour to 1 hour 10 minutes; if the edges of the crust brown too quickly, cover them loosely with foil. Transfer the pie to a wire rack to cool.

For the topping: Heat the butter in a medium skillet over medium heat until melted. Stir in the brown sugar, vanilla and salt, stirring until the mixture starts to bubble. Stir in the pecans and cook until the nuts are toasted and have absorbed most of the buttery sugar mixture, 3 to 5 minutes. Remove from the heat and gently stir in the coconut. Spoon over the pie and press gently to adhere to the filling. Let stand at least 20 minutes to cool. Serve the pie with whipped cream if desired.